The present disclosure relates to an external packaging material for covering a battery device, a nonaqueous electrolyte secondary battery using the same, and a battery pack. More particularly, the present disclosure is concerned with an external packaging material for battery device, which is made of a specific thin laminated film using no metallic foil, and which is advantageous not only in that it can reduce the thickness of the external packaging structure for a battery device in a nonaqueous electrolyte secondary battery, but also in that it can improve the battery in discharge capacity, a nonaqueous electrolyte secondary battery and a battery pack using the same.
In recent years, various types of portable electronic devices, such as videotape recorders (VTRs) with camera, mobile phones, and laptop computers, are widely used, and those having smaller size and weight are being developed. As the portable electronic devices are miniaturized, demand for battery as a power source of them is rapidly increasing, and, for reducing the size and weight of the device, a battery for the device must be designed so that the battery is lightweight and thin and the space in the device can be efficiently used. As a battery that can meet such demands, a lithium ion battery having a large energy density and a large power density is preferred.
Especially, batteries having high selectivity of shape, sheet-type batteries having a reduced thickness and a large area, or card-type batteries having a reduced thickness and a small area are desired, but the method having been employed in which a metallic can is used in the external packaging for battery makes it difficult to produce a battery having a reduced thickness.
For solving the problem, studies are made on batteries in which no liquid electrolyte is present by adding a substance having an ability to set the solution to an electrolytic solution or employing a gel electrolyte composed of a polymer. In these batteries, the adhesion between the electrode and the electrolyte is so strong that the contacts between the battery elements can be kept. Accordingly, it is enabled to fabricate a thin battery using an external packaging material in the form of film, such as an aluminum laminated film.
FIG. 1 shows the appearance of a battery using a laminated film as an external packaging material. This battery 1 is prepared by covering with a laminated film a battery device including a positive electrode and a negative electrode which are stacked with a separator disposed in between and spirally wound or zigzag folded, and sealing the laminated film around the battery device. In the battery 1, a positive electrode terminal 2a and a negative electrode terminal 2b (hereinafter, frequently referred to as “metal terminal 2” unless otherwise specified) connected to the positive electrode and the negative electrode, respectively, are electrically introduced from the battery through sealed portions of the laminated film. It is noted that, for improving the adhesion to the laminated film 4 for covering the battery device, the positive electrode terminal 2a and the negative electrode terminal 2b are respectively coated with sealants 3a and 3b which are resin leaves composed of maleic anhydride-modified polypropylene (PPa) or the like.
The laminated film used in the external packaging for battery is a multilayer film having moisture barrier properties and insulation properties, which includes a metal layer sandwiched between an outer resin layer and an inner resin layer each composed of a resin film. The metal layer has the most important role in preventing penetration of moisture, oxygen, or light to protect the contents of the battery, and, from the viewpoint of lightweightness, excellent elongation, low cost, and good processability, aluminum (hereinafter, frequently referred to as “Al”) is the most commonly used. As the outer resin layer, from the viewpoint of good appearance, high toughness, and excellent flexibility, nylon or polyethylene terephthalate (hereinafter, frequently referred to as “PET”) is used. The inner resin layer is a portion to be melted and sealed by heat or ultrasonic waves, and hence polyolefin is appropriate, and casted polypropylene (hereinafter, frequently referred to as “CPP”) is generally used. Bonding agent layers are formed between the metal layer and the outer resin layer and between the metallic foil and the inner resin layer.
For achieving a battery capacity as large as possible, it is necessary that a battery device as large as possible be contained in a predetermined outer size, and therefore it is desired that the external packaging for battery has a reduced thickness. For preventing penetration of moisture, oxygen, or light to protect the contents of the battery, the aluminum layer must prevent light penetration even during the shaping or processing, and therefore the aluminum layer needs a thickness of 30 μm or more, and has the largest thickness among the constituent layers of the laminated film. Accordingly, if the resin covering both sides of Al has barrier properties equivalent to or higher than those of Al, the Al layer is not required, enabling the most effective reduction of the thickness of the laminated film.
In the past, not only Al but also the resin cross-section of the sealed portion, especially in the battery structure as shown in FIG. 1, the sealed portion of the laminated film 4 through which the metal terminal 2 is electrically introduced are required to have the barrier properties of the external packaging. In covering the battery device with the laminated film 4 and sealing the laminated film around the battery device, the inner resin layer is melted and bonded, but the adhesion between the metal terminal 2 electrically introduced from the battery device and the inner resin layer is poor. Therefore, moisture is likely to penetrate the battery through the interface between the metal terminal 2 and the laminated film 4. If moisture penetrates the battery, an unfavorable electrochemical reaction occurs in the battery, so that the battery properties become poor or gas is generated in the battery to cause battery expansion.
For solving the problem, in Japanese Patent Application Publication (KOKAI) No. 2001-297736 and Japanese Patent Application Publication (KOKAI) No. 2004-039358, a method in which an auxiliary member is disposed between the metal terminal and the laminated film to improve the sealing properties is disclosed.
In the invention described in the Japanese Patent Application Publication (KOKAI) No. 2001-297736, the sealing properties are improved by bonding at least a linear low-density polyethylene film to the inner resin layer in the laminated film through a bonding agent and disposing an acid-modified, linear low-density polyethylene film as an auxiliary member between the metal terminal and the laminated film. Among thermoplastic resins, polyethylene has low moisture penetration, and hence can prevent moisture penetration through the auxiliary member.
In the invention described in the Japanese Patent Application Publication (KOKAI) No. 2004-039358, moisture penetration is prevented by using a thermoplastic resin containing a moisture absorbing agent, such as silica gel, activated carbon, or zeolite, in part of or whole of the auxiliary member. In Japanese Patent Application Publication (KOKAI) No. 2004-039358, there is a description of an example of auxiliary member such that, for securing the sealing properties, a reduced amount of the moisture absorbing agent is dispersed at a portion near the end of the laminated film through which the metal terminal is electrically introduced, and an increased amount of the moisture absorbing agent is dispersed at an inner portion of the battery.
Further, in the inventions described in Japanese Patent Application Publication (KOKAI) No. 11-086808 and Japanese Patent Application Publication (KOKAI) No. 11-007921, at the resin cross-section of the sealed portion other than the terminal portion, a moisture absorbing material, such as magnesium sulfate, is dispersed in a hot-melt resin inside of the Al layer to prevent moisture penetration through the sealed interface.
In Japanese Patent Application Publication (KOKAI) No. 2002-025511, a problem of further moisture penetration through the inner hot-melt resin is solved by a method in which a metal deposited film is formed on the resin surface.
In the above-mentioned techniques, moisture penetration through the cross-section of the sealed portion can be prevented. However, moisture penetration through the covering resin having no Al cannot be prevented. In addition, it is very difficult for the moisture absorbing agent to continue to absorb moisture for a long term. Further, the metal deposited film formed in the laminated film is easily broken during the processing (e.g., shaping) thus making it difficult to prevent moisture penetration.